Apparatus for detecting aqueous environments

ABSTRACT

A system for detecting the presence of a target measurand, such as water or specified chemicals which are carried in an aqueous solution, and which includes a fibre-optic probe assembly which incorporates an optical fibre, a thin film of a water swellable hydrogel, and a rigid containment structure. The hydrogel is in contact with the optical fibre such that a volumetric change in the hydrogel causes a microbend of the optical fiber. The microbend is detected by a sensor assembly which is coupled to the probe assembly.

BACKGROUND OF THE INVENTION

This invention relates to a detection system for use in detecting thepresence of a particular or target measurand. The system has particularutility in the detection of target measurands which are transported byaqueous means. One example is the ingress of aqueous measurands into theinterior of post-tensioned reinforced concrete sections.

Many large structures, such as road bridges, are constructed usingpost-tensioned reinforced concrete sections. This construction techniquerequires that steel tendons be inserted through ducts which run throughthe length of a concrete section. These tendons are tensioned in orderto apply a compressive load to the concrete. The remaining area withinthe ducts is filled with cement-based grout which forms a seal aroundthe tendons intended to prevent the ingress of moisture and de-icingsalts which would otherwise promote corrosion of the steel, thusweakening the structure. However, in structures such as road bridges,settlement and the vibrations produced by traffic tend to crack theconcrete and the grout, allowing water and other substances (e.g.chlorine) to reach the tendons, which are then subject to corrosion.Replacement of such corroded tendons is a difficult, and very expensivetask.

Another example where detection of aqueous measurands is desirable iswithin electrical or optical fibre cable duct systems where there may bea danger of corrosion penetrating the insulation material and givingrise to an electrical short circuit fault, or to loss of opticaltransmission. Aqueous measurands may also be measured in situations ofloss thereof from a container system (e.g. loss of fluid from apipeline) or as a measure of quality of a fluid in a pipeline. However,the measurand need not be borne directly or indirectly by aqueous means,for example it may be irradiation borne.

It is an object of the present invention to provide a detection systemwhich is capable of detecting the presence of such measurands, and thusallow repairs or other control measures to be carried out.

SUMMARY OF THE INVENTION

According to the present invention there is provided a detecting systemfor use in detecting the presence of a target measurand, said systemcomprising:

a fibre-optic probe assembly incorporating an optical fibre which issubject to micro bending at intervals along its length, the probeassembly comprising a rigid containment structure which is filled withthe length of optical fibre and a body of material which is subject to avolumetric change on exposure to said target measurand; and

a sensor assembly coupled to the probe assembly, the sensor assemblyhaving optical signal transmitting and receiving means arranged toidentify optical fibre microbend changes arising, in use, from forcesimposed locally on the fibre by the interaction of the rigid containmentstructure and volumetric changes in the body of material, characterisedin that said body of material comprises a hydrogel based polymer.

The system may be used simply to detect the presence of a targetmeasurand at an unspecified location along the length of the opticalfibre, though it is preferred that said signal transmitting andreceiving means is also capable of detecting the particular location onthe optical fibre where the signal carrying property of the opticalfibre has changed and thus also detecting the particular location of theaffected portion of the body of material (which is preferably elongate).

In a modification the system may have more than one probe assembly inwhich case the sensor assembly is provided with a logic function outputcircuit to decipher the effects of different target measurands on thedifferent hydrogel based polymers of the different probe assemblies.

The body of material may be extended to form a continuous rod or may bedeposited as a coating on a former. Preferably the thickness of thecoating is less than 50 microns.

The body of material may expand on exposure to one target measurand, andmay contract on exposure to another target measurand to be detected.

The exposure of the body of material to a target measurand may result ina permanent change in volume, or the material may return to its originalconfiguration on removal of the target measurand.

Preferably also, the hydrogel based polymer has chemical characteristicstailored to provide responsiveness of the target measurand.

The specific target measurands to be detected may be selected from pH,selected ions, certain chemicals, all of which are transported byaqueous means and photo-irradiation and temperature.

The body of material may comprise a second component, said secondcomponent being tailored to provide responsiveness to the targetmeasurand. The second component can either through interaction with thehydrogel, or alone, be subject to volumetric change on exposure to saidtarget measurand.

Preferably also, the containment structure comprises a sheath forexternally protecting the optical fibre and the body of material fromexternal disturbance, which sheath is porous to allow the body ofmaterial to be exposed to the target measurand. Alternatively, thesheath may be non-porous but sacrificially corrodible in the presence ofthe target measurand.

If the signal transmitting and receiving means produces pulses ofoptical energy into the optical fibre the backscattered energy resultingfrom such microbends may be measured as a function of time, or distancetravelled along the fibre length. This technique is known as OpticalTime Domain Reflectometry (OTDR), and an earlier application of OTDR isdescribed in EP A 0 490 849.

BRIEF DESCRIPTION OF THE DRAWINGS

The optical fibre is preferably bound to the body of material, whichpreferably is elongate, by an inelastic third member. Most preferably,the third member is thread-like in form. With this arrangement,expansion or contraction of a portion of the body of material causes thethread-like member to bite into, or relax its grip on the optical fibreat one or more distinct locations, thus increasing or decreasing themicrobends formed in the optical fibre by the thread-like member andfacilitating detection and location of the presence of the targetmeasurand.

The system has particular utility in post-tensioned reinforced concretesections. The system may be used during construction to ensure that theducts in which the steel tendons are located are properly filled withprotective grout, the system detecting the presence of the hydroxyl ionswhich migrate from the wet grout to the body of material. Thus, thesystem is actually being used to detect the absence of a specifiedcondition, that is a level of alkalinity, to indicate improper fillingof the duct with grout. In a finished section, the system may be used todetect the ingress of aqueous substances such as dissolved chlorine, orsalts, particularly de-icing road salt, which have the potential tocorrode or otherwise affect the steel tendon or the concrete.

Embodiments of the present invention will now be described by way ofexample, with reference to the accompanying drawings in which;

FIG. 1 is a somewhat schematic view of a detection system;

FIG. 2 is a graph showing the backscatter intensity profile of anoptical fibre of the system of FIG. 1 and showing microbend lossregions;

FIGS. 3A and 3B, are schematic views of a sensor cable showing differentmicrobend characteristics;

FIGS. 4A, 4B, 4C and 4D are perspective views of various embodiments ofsensor cables with restraining means attached;

FIGS. 5, 6 & 7A are views similar to FIG. 4 showing further embodimentsof a sensor cable;

FIG. 7B is a more detailed view of a feature of FIG. 7A; and

FIGS. 8A, 8B, 8C, 8D and 8E are side views of various embodiments of aflat sensor cable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 of the drawings illustrates a detecting system which comprises asensor assembly 10 coupled via an optical fibre 13 to a fibre opticprobe assembly 12 wherein a length of optical fibre 14, at least in use(as willbe explained), is subject to microbending at intervals along itslength. Fibres 13 and 14 are preferably identical. The sensor assembly10 includesan optical signal transmitter and receiver and operates as anOptical Time Domain Reflectometer (OTDR). The operation of the OTDR 10(described more fully in EP0490849) is essentially the following. Shortpulses of optical energy are launched into the optical fibre 13, whichis preferably multi-moded with a graded index core. As each pulsetravels down the fibrelength 14, energy is scattered and some of thisenergy is recaptured and guided back up the optical fibres 13, 14,towards the assembly 10 where itis detected and measured. Correlatingthe received backscatter energy with the time of launch of the pulseallows an estimation (curve A, FIG. 2) to be made of backscatter signalstrength as a function of position along theoptical fibre 14. Positionsof higher than usual loss i.e. arising from microbends can be identifiedand located along the optical fibre 14 length(as depicted at B in FIG.2). Within the probe assembly 12, the optical fibre 14 is arranged inassociation with a body of material 16 and a rigidcontainment structure18 in a geometrical configuration such that under exposure to themeasurand of interest on permeation of the structure 18, the opticalfibre 14, is mechanically disturbed, either creating a new microbend orchanging an existing microbend so as to exhibit a change in its losscharacteristics local to the point of measurand influence. In accordancewith the present invention the body of material 16 is selected tocomprise a hydrogel based polymer.

Hydrogels are three dimensional networks of hydrophilic polymers whichhavebeen tied together to form water-swellable but water insolublestructures. The term hydrogel is to be applied to hydrophilic polymersin a dry state (xerogel) as well as in a wet state. These hydrogels canbe tied together in a number of ways. Firstly, radiation or radicalcross-linking of hydrophilic polymers, examples being poly(acrylicacids), poly(methacrylicacids), poly(hydroxyethylmethacrylates),poly(glyceryl methacrylate), poly(vinyl alcohols), poly(ethyleneoxides), poly(acrylamides), poly(N-acrylamides),poly(N,N-dimethylaminopropyl-N'-acrylamide), poly(ethylene imines),sodium/potassium poly(acrylates), polysacharides e.g. xanthates,alginates, guar gum, agarose etc., poly(vinyl pyrrolidone)and cellulosebased derivatives. Secondly, chemical cross-linking of hydrophilicpolymers and monomers, with appropirate polyfunctional monomers,examples include poly(hydroxyethylmethacrylate) cross-linked withsuitable agents, the copolymerisation of hydroxyethylmethacrylatemonomer with dimethacrylate ester crosslinking agents, poly(ethyleneoxide) based polyurethanes prepared through the reaction ofhydroxyl-terminated poly(ethylene glycols) with polyisocyanates or bythe reaction with diisocyanates in the presence of polyfunctionalmonomers such as triols, and cellulose derivates cross-linked withdialdehydes, diepoxides and polybasic acids. Thirdly, incorporation ofhydrophilic monomers and polymers into block and graft copolymers,examples being block and graft copolymers of poly(ethylene oxide) withsuitable polymers,poly(vinyl pyrrolidone)-co-polystyrene copolymers,polyurethanes and polyurethaneureas and polyurethaneureas based onpoly(ethylene oxide), polyurethaneureas andpoly(acrylonitrile)-co-poly(acrylic acid) copolymers, and a variety ofderivatives of poly(acrylontriles), poly(vinyl alcohols) andpoly(acrylic acids). Fourthly molecular complex formation betweenhydrophilic polymers and other polymers, examples being poly(ethyleneoxides) hydrogel complexes with poly(acrylic acids) and poly(methacrylicacids). Lastly, entanglement cross-linking of high molecular weighthydrophilic polymers, examples being hydrogels based on high molecularweight poly(ethylene oxides) admixed with polyfunctional acrylic orvinyl monomers.

It is possible to produce hydrogels which are physically extremely weakwhen swollen in water so that they flow either under their own weight orunder low shear. However, the preferred hydrogels are characterized inthat when swollen fully with water they do not flow under their ownweight. Preferably they have also significant strength so that they cantransmit the osmotic pressure which develops within their structure whenthey swell in water. A further desirable but not essential feature isthatthey are tough and not brittle materials in the dry (or xerogel)non-hydrated state; that is xerogel materials which exhibit a glasstransition temperature (T_(g)) well below ambient temperature arepreferred. Preferably the T_(g) is below conceivable use temperaturesofthe cable. Many hydrogel materials have high T_(g) values well aboveambient temperatures and may be brittle and weak in use on extremeflexing. They can, however, be utilized as such extreme flexing israrely encountered.

A preferred group of hydrophilic polymer comprising hydrogels arePoly(ethylene glycols) (PEGs) based hydrogels either crosslinked or madeas chain-extended or block copolymers. Such crosslinked copolymers canbe made via the reaction of the hydrogel ends of the PEGs with adiisocyanateand a polyol. These are known as polyurethanes and aredescribed for example in UK Patent No. GB 2047093B, UK Patent No.1506473, European Patent Application Publication No. 0205815 andInternational Patent Application (PCT) Publication No. WO89/07117. Theblock copolymers of PEGscan also be made utilising only difunctionalunits such as, for example, a combination of poly(ethylene glycol),poly(propylene glycol), a diisocyanate and optionally a diamine.

A further preferred group of hydrophilic polymer comprising hydrogelsare based on linear chain-extended poly(ethylene oxide) polyurethaneureahydrogels (UK GB22354620) and a series of linear poly(ethyleneoxide)-co-poly(propylene oxide) block copolymer polyurethaneureahydrogels. These polyurethaneurea (PUU) materials are able to absorb andswell in aqueous media, while retaining their mechanical integrity. Thedegree to which the polymeric hydrogels will absorb and swell withaqueoussolutions is determined by the amount of hydrophilicpoly(ethylene oxide) (PEO) incorporated within their structures. Thehigher the PEO content, the greater the swellability of the hydrogelmaterial. The PUU hydrogels, when swollen, can have equilibrium aqueousmedia contents ranging from 5-95% by weight at ambient temperature. Thehydrogels also exhibit changesin swelling with variations in temperatureand may be described as "temperature responsive hydrogels."

As a result of their linear structure and chemical composition, the PUUhydrogels are soluble in a number of relatively "mild" organic solventssuch as methanol, ethanol, propan-2-ol, methyl ethyl ketone,dichloromethane and chloroform. The solubility of the PUU hydrogelsmeans that they can be readily fabricated into films or devices bysolvent casting techniques or used in coating applications. Theabsorption of aqueous media by the PUU hydrogels produces an increase intheir physical dimensions and this change can be used to exert amechanical force or pressure. The speed and extent of the swelling anddimensional response ofthe PUU hydrogels is determined by their degreeof hydrophilicity, governedby the PEO content, their physical dimensionsand to the temperature of thesystem. The poly(ethylene oxide) based PUUhydrogel systems have an inverseswelling response in aqueous media withincreasing temperature. A swollen PUU hydrogel will decrease in swellingas the temperature of the system isincreased. The decrease in swellingof the PUU hydrogel will result in a contraction of the physicaldimensions of the material which can be used to produce a mechanicalresponse. The hydrogels described can be manufactured by various knownprocesses and have the advantage that they are solvent soluble andtherefore can be made in a form suitable for coating. They are alsothermoplastic and may be extruded into fibres from the melt (with orwithout plasticizers). The nature of such materials is explained in"Polymer Science and Materials", Tobolsky, A. V. M. and Mark,H. F.,Wiley-Interscience 1971.

The poly(ethylene oxide) based polyurethaneurea hydrogels can be used incombination with poly(acrylic acids) or poly(methacrylic acids) toproducepH responsive hydrogels through the formation of macromolecular,hydrogen-bonded association complexes between the polyether and thepolyacid segments within the hydrogel structures. These materials aresoluble in solvent systems and are therefore suitable for the productionof pH responsive hydrogel films and coatings. It has been demonstratedthat the PUU/polyacid complexed membranes have a low to high swellingresponse at about pH4.0 in citrate/phosphate buffer systems over therangepH2.2-pH8.0.

The swelling behaviour of poly(acrylamide-co-acrylic acid) copolymergels in response to changes in ionic strength and pH, indicate thatswelling responses can also occur in environments of different ionicstrengths and at both low and high pH values. At very low pH thepoly(acrylamide-co-acrylic acid) gel will deswell to the volume of anunionised gel. As the pH is increased the gel will increase in swellingasthe acid groups become ionised until at high pH values, (>pH10), thegel begins to deswell due to the increased concentration of cationswithin thegel. It has been shown that the PUU/polyacid hydrogels displaythis type ofswelling response at high pH values(pH10-pH12).

A further group of hydrophilic polymers comprising Polymeric microgelshavebeen developed (UK patent GB2090264B), via a solution polymerisationprocess, comprising crosslinked particles which are capable of forming asol in the reaction solvent. These crosslinked particles or microgelscan be designed to have specific functionalities, reactivities,solubility andsize. Basicpoly(methylmethacrylate-co-(dimethylamino)ethylmethacrylate) microgelshave been developed which, when incorporated into a PUU hydrogelmatrixproduce pH responsive hydrogel materials which exhibit a change inswelling at about pH6-7. The versatility of the microgel process meansthat microgels can be prepared which will respond to any chosen externalstimuli. For example, microgels incorporating acidic and/or basic groupswill respond to changes in pH and/or ionic strength,poly(hydroxyethylmethacrylate-co-dinitrophenol)microgels will respond tothe presence of amines, poly(hydroxyethylmethacrylate-co-azobenzoate)microgels will respond to UV radiation and poly(N,N-alkyl substitutedacrylamides) based microgels will have a swelling response in relationto the system temperature.

These microgels can, in a two component system with a PUU hydrogelmatrix, or a carrier or binder produce responsive hydrogel materials,which swell or deswell i.e. shrink, on exposure to the specific targetmeasurand.

Numerous geometrical embodiments of the probe assembly 12 are possible,as will now be explained with reference to FIGS. 4-8.

In FIGS. 3A and 3B the fibre 14 is linearly co-located in intimatecontact with (or in very close proximity to) a hydrogel coating 16carried by a central supporting former 20 (which forms part of thecontainment system 18). The volumetric change of the hydrogel coating16, on exposure to the target measurand, is transduced into the requireddeformation of the optical fibre 14 by restraining the optical fibre 14from radial movement at regular intervals along the length of the fibre14 as indicated by the arrows. In FIG. 3A the restraining effect isimplemented symmetrically butin FIG. 3B it is implementednon-symmetrically. The preferred arrangement is a periodicity coincidentwith achieving resonant or near resonant powertransfer from a guidedmode in the optical fibre 14 to an unguided mode, oran integral multipleof such a periodicity.

Physical implementations of the FIG. 3A arrangement are depicted inFIGS. 4A and 4B, and of the FIG. 3B arrangement in FIGS. 4C and 4D. FIG.4A usesC-shaped clamps 22; FIG. 4B uses restraining O-rings 24; FIG. 4Cuses a single restraining wire 26 wound as a helix; and FIG. 4D usesmultiple restraining wires 28 wound as helices.

In FIG. 5 the optical fibre 14 is sandwiched between a body of hydrogel16 and a primary restraining braid 30 formed of a helically woundsuitable material (e.g. nylon, kevlar, steel) and a secondaryrestraining braid 32 of similar material. The secondary braid 32material may in some instancesalso serve as a chemically porousmechanical support.

FIG. 6 uses only the helical winding of the primary restraining braid30. In FIGS. 5 and 6, the applied stress is not uniform along theoptical fibre 14 length but is concentrated periodically at the positionof the individual windings of the restraining braid 30.

In FIGS. 7A and 7B a profiled coating 34 is applied to the optical fibre14. The coating is non-uniform along the optical fibre 14, by forexample,incorporating some particulates in the coating mix. The effectof the particulate is to act as a local stress concentrator. Thehydrogel 16 volumetric change exerts a uniform hydrostatic pressurewhich through the inhomogeneity of the coating 34 is transduced into amicrobend loss in thefibre 14.

In situations where a flat, or rectangular implementation is requiredany one of the arrangements shown in FIGS. 8A-E can be implemented. Inthese arrangements the optical fibre 14 is located between a planarrestraining plate 36, and a hydrogel layer 38, (cross-hatched) and abottom restraining plate 40. FIG. 8A uses the hydrogel layer 38 on theprofiled bottom restraining plate 40; FIG. 8B uses the profiled hydrogellayer 38 on the bottom restraining plate 40; FIG. 8C uses an additionalprofiled restraining plate 42 on the flat hydrogel layer 38; FIG. 8Duses an additional profiled restraining plate 44 on the top restrainingplate and FIG. 8E uses a restraining wire 46 on the optical fibre 14.

In all the probe assemblies 12 which have been described, the responsetimeand the magnitude of the response can be tailored through theappropriate selection of periodicity of disturbance, thickness of thehydrogel 16, stiffness of the hydrogel 16, stiffness of supportingformer 20 and or theuse of additional buffering layers.

For example, a probe assembly 12 as previously described containing ahydrogel 16 of the composition found in Example 1 gives a 0.04 dB/m lossin backscatter intensity for a hydrogel coating of 20 microns and a 0.1dB/m loss in backscatter intensity for a hydrogel coating of 45 microns.The time taken to detect a response is in the region of 15-20 seconds.

It will be understood that whereas current OTDR systems are capable ofdetecting changes in backscatter intensity of around 0.01 dB, for acalibrated change of between 0.01-0.05 dB/m (in the presence of thetargeted substance) probe assembly lengths of between 200 m and 1 km canbe interrogated, assuming a dynamic range of 10 dB.

The detection system of the present invention may be utilised as aquality assurance tool to ensure that grout extends through the entireduct of a post-tensioned reinforced concrete section. For such anapplication, the hydrogel 16 is selected to swell on exposure tohydroxyl ions, when the concentration of such ions exceeds apredetermined value, for example to produce a pH of 10 or above, whichlevel of ions is present in the wet grout. Thus the absence of grout atany point in the duct is detectable asan area of low loss, associatedwith localized non-swelling of the hydrogel

A modified form of the detection system may be used to monitor both thequality of grout filling and the subsequent ingress of moisture. Thissystem has twin probe assemblies with different hydrogels one beingsensitive to the pH levels of 6 and greater, and the other beingsensitiveto pH levels of 10 and greater. The outputs from the sensorassemblies are combined in a simple truth table to identify thecondition of the concretesection.

EXAMPLE 1 Synthesis of a Linear Block Copolymer PolyurethaneureaHydrogel.

Materials

    ______________________________________                                        dry Poly(ethylene glycol) PEG 5860                                                                    1       mol                                           dry Poly(propylene glycol) PPG 425                                                                    10      mol                                           4,4'-Methylenedianilene MDA                                                                           0.824   mol                                           4',4'-Dicyclohexylmethane diisocyanate -                                                              12.415  mol                                           (Desmodur W)                                                                  ______________________________________                                    

Catalyst=0.2 mg FeCl₃ /g total reactants.

Procedure

Dry PPG425 was mixed with the FeCl₃ catalyst and placed in an oven at95° C. until the catalyst had dissolved. The MDA was then added andthemixture returned to the oven, with occasional stirring, at 95° C. untilthe diamine had completely dissolved and the reaction mix was visiblyhomogeneous. Molten, dry PEG5860 was then added to the molten mixtureand stirred vigorously before being returned to the oven to equilibrateto 95° C. The Desmodur W was then added to the mixture rapidly, inside afume cupboard. The mixture was then stirred vigorously for 2 minutes andthe reacting melt was then transferred to preheated polypropylene mouldsand placed in the oven at 95° C. for 20 hours.The polymer was thenremoved from the moulds and swollen in water to removeany low molecularweight soluble fractions. The swollen polymer was then chopped intopieces and dried in air and then under vacuum to remove the water.

The PUU composition could then be dissolved in a 5% w/v solution of asuitable "mild" organic solvent, from which films and coatings could becast.

We claim:
 1. A detecting system for use in detecting the presence of atarget measurand, said system comprising:a fibre-optic probe assembly(12) incorporating an optical fibre (14) which is subject to microbending at intervals along its length, the probe assembly (12)comprising a rigid containment structure (18) which is filled with thelength of optical fibre (14) and a body of material (16) which issubject to a volumetric change in the presence of said target measurand,said body of material comprising a hydrogel based polymer formed from anorganic solvent soluble hydrogel and wherein said body of material is inthe form of a thin film having a dry thickness of less than 50 microns;and a sensor assembly (10) coupled to the probe assembly, the sensorassembly (10) having optical signal transmitting and receiving meansarranged to identify optical fibre microbend changes arising, in use,from forces imposed locally on the fibre (14) by the interaction of therigid containment structure (18) and volumetric changes in the body ofmaterial (16).
 2. A detecting system as claimed in claim 1 wherein thehydrogel based polymer comprises, radiation or radical cross-linkedhydrophilic polymers.
 3. A detecting system as claimed in claim 1wherein the hydrogel based polymer comprises, chemical cross-linkedhydrophilic polymers and monomers, with appropriate polyfunctionalmonomers.
 4. A detecting system as claimed in claim 1, wherein thehydrogel based polymer comprises, block and graft copolymers comprisinghydrophilic monomers and polymers.
 5. A detecting system as claimed inclaim 1, wherein the hydrogel based polymer comprises, hydrophilicpolymers molecularly complexed to other polymers.
 6. A detecting systemas claimed in claim 1, wherein the hydrogel based polymer comprises,entanglement cross-linked high molecular weight hydrophilic polymers. 7.A detecting system as claimed in claim 1, wherein the body of material(16) further comprises a non-responsive carrier or binder.
 8. Adetecting system as claimed in claim 1 wherein the hydrogel basedpolymer comprises, a microgel.
 9. A detecting system as claimed in claim1, wherein the containment structure (18) comprises a sheath which isporous to the target measurand.
 10. A detecting system as claimed inclaim 1, wherein the containment structure (18) comprises a sheath whichis non-porous but is reactive with the target measurand so as to becorroded in the presence of the target measurand.